Analysis of stabilization of photon beam softening with off-axis distance for filtration system enhancement to increase dosimetry in radiotherapy

Abstract

Beam softening is depending on the material atomic number and photon beam energy for photon beam filtration; the filtration quality is essential to producing a clinical beam in external radiotherapy. Beam softening analysis is done using Monte Carlo method. The Monte Carlo model was performed for 6 MV photon beam produced by Varian Clinac 2100 with flattening filter thereafter the flattening filter was removed and replaced by slab of aluminum and copper with different thickness. The purpose of this study is to analyze the photon beam softening with off-axis distance for the reference field size of 10 × 10 cm2; the beam softening is studied in terms of tow coefficients a1 and a2.Inside the irradiation field, for aluminum slab, the coefficient a1 varied in interval from −1.2 cm−1 to 1.7 cm−1 and it has big fluctuations with off-axis distance. For copper, a1 varied from 0 cm−1 to 1.5 cm−1 and it has small fluctuations with off-axis distance. The coefficient a2, for aluminum slab, varied from −1 cm−2 to 0.4 cm−2 with big fluctuations. For copper slab, a2 varied from −0.9 cm−2 to 0 cm−2 with small fluctuations. We conclude that the beam softening is very stabilized for copper than aluminum in despite of copper attenuate more photons than aluminum for same slab thickness when they are used in photon beam filtration. To increase dosimetry in radiotherapy, the flattening filter should be constructed based on low atomic number materials but with high stabilization of beam softening with off-axis distance. Keywords: Material filtration, Monte Carlo simulation, Photon beam softening, Material study, BEAMnrc